1. Field of the Invention
This invention relates to a carburetor for use with an internal combustion engine and, more particularly, to a variable venturi carburetor having a variable venturi for variably determining the opening of a venturi throat defined thereby.
2. Description of the Prior Art
Normal variable venturi carburetors include a through passage-way mixture conduit with oppositely disposed inlet and outlet ends. The mixture conduit has a variable venturi for variably determining the opening of a venturi throat defined thereby, and a throttle plate located within the outlet end of the mixture conduit for opening and closing the mixture conduit and thereby metering the amount of mixture that exits from the mixture conduit. A fuel nozzle opens into the mixture conduit at a point on the downstream side of the variable venturi and connected to a fuel reservoir for providing fuel in amounts dependent on vacuum appearing and the mixture conduit between the variable venturi and the throttle valve. A jet needle is inserted into the fuel nozzle and drivingly associated with the variable venturi for controlling the amount of fuel drawn from the fuel nozzle so as to maintain the ratio of the airfuel mixture constant.
Another type of variable venturi carburetors are also well known in which the jet needle is removed and instead an electromagnetic valve is provided in the fuel condiut leading to the fuel nozzle for opening and closing the fuel conduit so as to control the amount of fuel discharged from the fuel nozzle. The variable venturi carburetors of this type further include a sensor for detecting the opening of the venturi throat and a control circuit associated with the opening sensor for providing a drive pulse signal with a duty factor corresponding to the opening of the venturi throat to the electromagnetic valve to control the amount of fuel flowing through the electromagnetic valve.
Although the latter variable venturi carburetors boast several advantages over the former variable venturi carburetors in that their control accuracy is subject to less variation with time and their size is relatively small, they present other difficulties. First, they require a large electromagnetic valve capable of passing a large volume of fuel so as to control the inlet air-fuel ratio over the full range of operation of the engine. Second, high techniques are required to control the operation of the electromagnetic valve with a drive pulse signal having its duty factor varying in a wide range. Third, a high accurate sensor is required which can detect the amount of air flowing through the variable venturi with high accuracy. These difficulties stem mainly from the arrangement in which the electromagnetic valve is used to control the amount of fuel drawn from the fuel nozzle over the full range of operation of engine.